HackerRank - Common Child

#include <cmath>
#include <cstdio>
#include <vector>
#include <unordered_set>
#include <string>
#include <iostream>
#include <algorithm>
using namespace std;

int main() {
    string a, b;
    cin >> a;
    cin >> b;
    size_t len = a.length();
    vector<vector<int>> dp(len + 1, vector<int>(len + 1, 0));

    for (int i = 0; i <= len; i++)
    for (int j = 0; j <= len; j++)
    {
        if (i * j == 0)    dp[i][j] = 0;
        else if (a[i - 1] == b[j - 1])
            dp[i][j] = dp[i - 1][j - 1] + 1;
        else
            dp[i][j] = max(dp[i - 1][j], dp[i][j - 1]);
    }

    cout << dp[len][len];
    return 0;
}